Production of stainless steel



Patented Dec. 21, 1948 UNITED STATES PATENT OFFICE mesne assignments, toArmco tion, a corporation of Ohio Steel Corpora- No Drawing. ApplicationMay 21, 1945, Serial No. 595,063

Claims. 1

My application is a continuation-in-part of my copending applicationSerial No. 432,492 filed February 26, 1942 now abandoned and entitledProduction of stainless steel and the invention relates to theproduction of stainless steel and more especially to a method of lowsulphur content.

Among the objects of my invention, is the provision of a safe, practicaland eflicient method of producing stainless steel, which includes theuse of high-sulphur raw material; which in a simple, direct andeconomical manner nevertheless gives a clean, finished productsubstantially free of sulphur; which avoids the introduction of harmfulingredients into the metal during the elimination of sulphur; which ishighly effective and quite simple to perform; and which in performancerequires the use of cheap and readily available materials.

Other objects in part will .be obvious and in part pointed outhereinafter.

The invention accordingly, consists in the combination of materials andcomposition of ingredients, and in the several steps and the relation ofeach of the same to one Or more of the others as described herein, thescope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of myinvention, it should be noted at this point that stainless steel isdefined as a low-carbon steel comprising to 35% chromium,'with orwithout nickel, and with or without supplemental additions of manganese,silicon; cobalt, copper, molybdenum, tungsten, vanadium, columbium,titanium, sulphur, and the like, for special purposes, and the balancesubstantially all iron.

In accordance with certain practices, it is customary to prepare a meltof plain low-carbon steel of high purity, and to incorporate chromium inthe melt by way of expensive refined ferro-alloys, such as low-carbonferrochrome. Recently developed processes of producing stainless steel,

however, involve the use of cheap raw materials as a source of chromium,and are directed toward the achievement of a lower ingot cost. Theseprocesses broadly include melting in proper proportions, a cheapchromium-bearing ingredient and a source of iron in a suitablemetallurgical furnace, to produce an iron-chromium bath. The bath issubjected to refining treatment whereby carbon is removed and a finishedsteel is produced.

In the more recent processes, the raw materials serving as a source ofchromium include stainless steel scrap, which becomes available in largequantity about the melt shop, the rolling mill, and the various customerplants, in the form of pit and skull scrap, crop-ends, cuttings, and thelike. Likewise, in certain more recent processes, the source of chromiumincludes chrome ore and/or high-carbon ferrochrome, the proportions ofthese ingredients and stainless steel scrap being adjusted to achievesatisfactory furnace operating conditions, and also to recognize varia-'tions in the availability of these materials and,

the fluctuation in their market prices.

Particularly in the more recent stainless steel processes, a problem ofeliminating excess sulphur has arisen. This comes about through thewidespread use of the free-machining grades of stainless steel. Thesesteels purposely have a high sulphur content to give the free-machiningcharacteristic. The scrap from these high sulphur steels, for example,in the form of scrap metal, roll scale, grinding dust, and the like,provide large quantities of iron and chromium. They possess limitedvalue in stainless steel production because of their high sulphurcontents. Unfortunately, however, it is impossible or commerciallyimpractical to exclude these from the scrap yard, particularly since anumber of different grades of steel are made and rolled at a plant andthere is no feasible way of separating the scrap. Where such rawmaterials are employed, large quantities of sulphur go into the finishedsteel as an incidental ingredient, and this in many instances proves tobe a serious matter.

Although sulphur serves a beneficial function in certain grades ofstainless steel, as in the "freemachining grades, control over sulphuris important where an excess is to be avoided or where steel of certainquality is desired, as in other grades. Stainless steel containingabnormally high percentages of sulphur is characterized, for example, byhot-shortness and thus the metal is inclined to tear in hot-working. Thesulphur promotes fibrous structure which causes splitting in hot-workingand in cold-forming, as in cold-heading upsetting and the like.Moreover, excess. sulphur reduces the normal ductility of stainlesssteel, and corrosion resistance of the steel in certain media isadversely aiiected.

I find that the elimination of sulphur from a melt of stainless steelinvolves considerations en'- tirely different from those encountered inthe removal of sulphur from plain carbon steel. In way of illustration,the conventional carbide slag process employed in removing sulphur frommolten carbon steel, cannot be applied successfully low-carbon to moltenstainless steel. Chromium possesses a great thirst for carbon, and thecarbon available in the carbide slag therefore, goes into solution withchromium of molten stainless steel. It must be noted in this connectionthat carbon, as well as other impurities picked up by the refined moltenmetal, as for example, after a metal deoxidizing operation, is a seriousmatter for the properties of the metal are adversely affected.

Moreover any elimination of incidental quantities of sulphur in priorprocesses of stainless steel wherein the conventional lime-fluorsparslag' is employed is not applicable to the elimination of substantialquantities of sulphur in the presence of chromium. I find that the slagis objectionably heavy. Moreover that it quickly loses its fluidityapparently due, at least in part, to a decomposing of fluorspar, thisgiving a higher lime content which then must be compensated with afurther addition of fiuorspar. Furthermore, I find that in hightemperature furnace operations the odor of fluorine is objectionablynoticeable about the melt-shop. This is particularly objectionable tothe crane operators. men and equipment.

An object of my invention, therefore, is the production of stainlesssteel of low sulphur content in a simple, direct and economical manner,using raw materials which contain substantial amounts of sulphur, whichmethod of production avoids introducing into the metal undesirablecontaminating materials such as carbon, and which in addition toeflecting highly satisfactory removal of sulphur, achieves sound, cleanmetal, free of undesirable metal oxides.

Referring now more particularly to the preferred practice of myinvention, I employ an electric arc furnace of the well known Heroulttype, lined to a height somewhat above the-slag line with chromitebrick, and having side-walls and roof of silica brick. Either carbon orgraphite electrodes may be employed, the applied voltage beingpreferably available in several steps within the range of 100 to 275volts. In preparing the furnace for reception of a charge, I apply aprotective coating of burnt lime over the chromite brick lining. Theproportion of lime used approximates pounds per ton of steel to beproduced in the furnace, and serves to retard erosion of the lining bymolten metal and slag.

In the production of a heat of stainless steel, I charge into thefurnace as a source of chromium, one or more of the group consisting ofchrome ore, high-carbon ferrochrome, and stain- It is unsafe to.

less steel scrap. This latter, such as imperfect ingots, ingot butts,crop ends, roll scale, pit and skull scrap, grinding dust, and the like,may contain considerable amounts of the sulphur-bearing orfree-machining grades of stainless steel. An amount of available oxideof iron, such as plain low-carbon steel roll scale, suflicient to removeand/or exclude carbon entering into the process and an amount ofordinary base iron or steel scrap, also are conveniently charged withthe 4 begins to melt down. There is formed a ferrous metal bathcontaining chromium. Overlying this bath, there is formed a blanket ofslag which is strongly oxidizing in character, by virtue of the largequantities of iron oxide, and chromium oxide in the charge.

The strongly oxidizing slag blanket is instrumental in oxidizing thecarbon supplied the molten metal by the furnace charge, Carbon in thefurnace atmosphere and furnace electrodes, generally is excluded fromthe melt by the slag blanket. Any such carbon picked up by the bath,however, is oxidized along with that coming from the charge. Theoxidation of carbon from the metal is greatly accelerated and renderedeffective in achieving a commercial practical process by conducting themelting operation at a high melt-down temperature.

No reliable method is known to me for determining accurately thetemperature of the metal bath under the slag blanket, but it isestimated that this temperature should be approximately 3050 F. to 3200F., which is some 100 to 250 F. higher than is usually employed inprocessing carbon steels. The high temperature which I employ rendersthe iron oxide slag more active in combining with carbon, and thusrenders more effective the removal and exclusion of carbon from themelt. After melting down, additional mill scale may be added to theslag, as circumstances require. Samples are taken of the metal and whenthe carbon content is found-to be sufliciently low, usually 0.1% orless, the oxidation period is ended.

I then carry out a second or reducing step, wherein recovery is had ofthe reducible oxide content of the slag. I perform the reducing step byadding to the bath a non-carbonaceous reducingagent, such as crushedferrosilicon, alsifer (aluminum-silicon-iron alloy), or crushedferrochrome-silicon, in amount chemically in excess of the oxides ofiron and chromium present in the slag. Chromium oxides and iron oxidesin the slag are reduced by the non-carbonaceous reducing agent and thechromium and iron gravitate into the melt. Silicon contamination of themelt is prevented during this step by adding burnt lime to the melt inamount ranging from 3 to 5 times that of silicon in the non-carbonaceousreducing agent. Conveniently, the burnt lime and non-carbonaceousreducing agent are mixed together in proper proportions, on the floor ofthe 'to a relatively low value.

Following the reduction period, I withdraw the slag overlying the metalbath. In order to ascertain the amount of sulphur picked up by themolten metal from the materials of the furnace charge, I promptly takesamples of the metal and run a sulphur test on the samples. Ordinarily,these tests show an excessive sulphur content, for example, .03'% to.10%, while the specifications require under .030% and usually under0.015%, as large rejections have been experienced, when sulphur is over.015 on many types of stainless steel.

I then build up a sulphur reactive slag on the surface of thehigh-sulphur metal bath. The slag which I preferably employ containslarge amounts of burnt lime ranging in amount. de-

pending upon sulphur content of the metal and ilmenite, rutile,titanite, or other titanium-oxide bearing ingredient, in amountsufficient to render the slag thin, fluid and cream in appearance. Thetitanium-oxide does not decompose but directly serves to fluidify theslag. Because of this permanence there is no loss of fluidity as thereaction progresses.

In my sulphur-reactive slag I employ about 55 to 75 pounds of burnt limeper ton of metal, and preferably around 65 pounds per ton. These rangesare in general critical because I find that with a lesser amount oflime, sulphur is not elimihated and with a higher amount the slag tendsto stiffen and become thick and difficult to manage thus precludingsulphur elimination. The ilmenite, rutile or titanite usually isemployed in the amount of about to 30 pounds per ton of metal, about 20pounds per ton of metal being an average addition. This addition is notquite so critical as that of the lime. The amount of (ilmenite, or othertitanium-oxide bearing ingredient added need be suiflcient only toestablish proper slag conditions as noted and this generally is had withsome 10 to 30 pounds of ilmenite per ton of metal, the ilmenitegenerally amounting to between A; and of the lime employed.

In the practice of my process, I find it advantageous to build up theslag in stages, as for example, by adding to the metal about one-halfthe lime and titanium-oxide bearing ingredient immediately after thesulphur test. When the first added slag ingredients are rendered fluidunder the intense heat of the molten metal, I add the balance of thelime and titanium-oxide-containing ingredient, in proper proportion as amixture, reserving separate amounts of each ingredient for separateaddition to, the melt where conditions require.

Once the sulphur-reactive slag blanket is provided on the molten metal,which usually requires about one-half hour, it is held on the metal fora period of about three-quarters of an hour. During this latter period,the slag and the metal are rabbled from time to time, as for example, atfive-minute intervals. The rabbling serves to maintain the slag in fluidcondition, and also brings the slag into more intimate contact with themolten metal.

Apparently, in my process, sulphur in the metal reacts with the verylarge amounts of lime present in the thin, fluid slag. It is believedthat calcium sulphide accordingly forms a slag ingredient. Check testsare made throughout the finishing period, in order to ascertain thesulphur content of the metal, as well as other ingredients. A great lossof sulphur is observed.

Where the tests indicate metal of proper analysis, I then tap thefurnace into a ladle for teeming. From the ladle, the melt is teemedinto ingot molds, where it is permitted to solidify and cool. Aftercooling, the ingots are stripped from the molds and are ready forstorage and subsequent reheating and conversion.

Where desired, during the finishing period, supplementary additions ofchromium, nickel, copper, tungsten, vanadium, aluminum, titanium,zirconium, and the like, are made, as desired, either in the furnace orin the ladle in order to bring the steel to final analysis. Theadditions usually are made by the use of refined ferro-alloys or byemploying electrolytically-derived metal.

My process, accordingly, advances the art of stainless steel productionto a point where high sulphur ingredients are sucessfully used as asource of chromium and/or iro in making lowsulphur stainless steel.Asnote above, my process enables the consumption of high-sulphurstainless steel waste, including pit and skull scrap imperfect ingots,ingot butts, crop ends, roll scale, grinding dust, and the like. whichheretofore had limited reclamation value. Since large amounts of mixedwaste including high sulphur steel becomes available in the melt shop,and in the various consumer plants where rustless steel sheet, strip,bars, rods, wire and the like are fabricated, my process hasconsiderable value from the standpoint of economy. It will, of course,be understood that; certain ore, plain carbon steel waste, and the like,also serve as a source of objectionable sulphur in the production ofstainless steel, and that my process is employed to advantage inconsuming such materials.

The desulphurizing step of my invention is simple to perform and inperformance requires only cheap and readily available materials. Withthe step, a considerable loss of sulphur is successfully achieved in ashort period of time. I find, for example, that steel which afteroxidation and reduction treatement, contains 0.025% to 0.030% sulphur,is refined to a sulphur content of 0.002% to 0.010% in about one hourand fifteen minutes. This period is that from the slag-off stepfollowing reduction, to the time of tapping.

In the practice of my invention, I find that the desulphurizing slagalso acts as a protective blanket for the molten metal. The rabbling,and

the ilmenite, rutile, titanite slag ingredient ensure that a fluid slagconsistently is maintained throughout the period of metal treatment. Themetal bath, therefore, is not exposed appreciably to the atmosphere andreadily oxidizable alloy constituents, such as chromium, in the metal,are little affected.

The consistently fluid condition of the finishing slag, as well as therabbling operation, ensure most intimate contact between slagingredients and the molten metal. There is no loss of fluidity throughdecomposition and the desired proportions of lime and titanium-oxidebearing ingredients readily maintained throughout the operation. Sulphurin the metal thus is thoroughly eliminated by reaction with the largequantities of burnt lime in the slag. The slag, moreover, is ofnon-carbonaceous character and the melt, therefore, is not contaminatedwith extraneous carbon.

Thus it will be seen that there has been provided in my invention, anart of producing lowsulphur stainless steel, in which tl're variousobjects hereinbefore noted, together with many thoroughly practicaladvantages are successfully achieved. It will be seen, further, that theprocess lends itself to the rapid, efficient, economical and reliablemanufacture of low-sulphur stainless steel of high purity employing amaximum of available and inexpensive sulphur-containing raw materials.The process is particularly favorable to achieving economy, throughconsumption of a wide range of chromium-containing ingredients;especially scrap, roll scale, grinding dust, high-carbon ferrochrome andchrome ore, which are combined proportionately, with practical limits,in accordance with the variations in their availability, and inaccordance with the fluctuations in their market values.

While as-illustrative of the practice of my invention, one or morechromium-bearing ingrediperiod, where practical, rather than as aningredient in the initial furnace charge.

As many possible embodiments may be made of my invention and as manychanges may be made in, the embodiment hereinbefore set forth. it willbe understood that all matter described herein, is to be interpreted asillustrative, and not in a limiting sense.

I claim: I

1. In the production of stainless steel of low carbon and low sulphurcontents, the art which includes, melting in an electric arc furnacehaving a chromite lining a charge including highsulphur stainless steelscrap and high-carbon ferrochrome thereby forming a bath of highsulphurmetal; maintaining said bath under an oxidizing slag blanket. at a .hightemperature to remove carbon from the bath, this action beingaccompanied by a loss of chromium into the slag; adding an excess ofsilicon-containing reducing agentto the slag whereby chromium containedin the slag is precipitated into the underlying bath; removing theresidual slag from the surface of said bath; building up on the bath asulphur-reactive lining finishing slag comprising substantial amounts ofburnt lime and ilmenite: and then rabbling the slag and bath to effectremoval of sulphur from the bath while minimizing contamination withchromite.

excess of silicon-containing reducing agent to:

the slag whereby chromium contained in the slag is precipitated into theunderlying bath; removing the residual slag from the surface of saidbath; building up on the bath asulphur-reactive lining finishing slagcomprising substantial quantities of burnt lime and a slag-fluidiiyingagent largely consisting of titanium-oxide; and then rabbling the slagand bath to effect removal of sulphur from the bath while minimizingcontamination with chromite.

3. In desulphurizing in an electric arc furnace having a chromite lininga bath of stainless steel of low carbon content and containing an excessof sulphur, the art which includes, preparing on the clean surface ofthe bath as a finishing step a sulphur-reactive basic slag comprisingapproximately 55 to 75 pounds of burnt lime per ton of metal in saidbath, and substantial amounts of a titanium-oxide containing ingredient,and rabbling said slag and bath thereby eliminating the sulphur in thepresence of chr0- mium without carbon contamination and withoutoxidation and chromite contamination of the bath. a

4. In the production of stainless steel of low carbon and low sulphurcontents, the art which includes, melting in an'electric arc furnacehaving a chromite lining a charge comprising stainless steel scrap,high-carbon ferrochrome and chrome ore, in which charge sulphur ispresent in substantial amount, thereby giving a substantially highsulphur metal bath with a slag containing the'oxides of iron andchromium; reducing the oxides of iron and chromium of the slag with anexcess of silicon-containing reducing agent to eflect a recovery of itsiron and chromium values; removing the residual slag; and thendesulphurizing the bath, while minimizing chromite contamination, bymeans 'of a fluid basic finishing slag comprising substantial amounts ofburnt lime and "a titanium-oxide containing ingredient charged onto thebath in batches to give and maintain a slag of creamy consistency.

5. In the productionof stainless steel of low carbon and low sulphurcontents, the art which includes, melting in an electric arc furnacehaving a chromite brick lining a charge including stainless steel scrapand an oxidizing slag-forming material, in which charge sulphur ispresent in substantial amount, to form a slag covered bath of ferrousmetal containing chromium and an excess of carbon andsulphur;maintaining said bath beneath the slag at high temperature untilthecarbon content of the bath is lowered to desired value, said stepbeing accompanied by a loss of chromium into the slag; reducing theoxides of iron and chromium of said slag with an excess ofsilicon-containing reducing agent, thereby achieving a recovery ofchromium lost from said bath; withdrawing the remaining slag from saidbath to give a clean bath surface; and immediately desulphurizing thebath, while minimizing contamination of the metal with chromite, bymeans of a finishing slag containing at least 55 pounds of burnt limeand at least 10 pounds of ilmenite per ton. of metal bath.

DONALD L. LOVELESS.

REFERENCES CITED The following references are of, record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 768,265 Carlsson Aug. 23, 19041,458,568 Bennett June 12, 1923 1,662,977 Shackelford et al. Mar. 29,1927 1,954,400 Arness Apr. 19, 1934 2,049,091 Stimson July 28, 19362,110,066 Heuer Mar.- 1, 1938 OTHER REFERENCES Refining MetalsElectrically, pages 123 to 126, by Barton; published in 1926 by thePenton Publishing Co., Cleveland, Ohio.

